More than 50 years after the total solar eclipse of 1955, Lankan star gazers, have a
not- to -be- missed opportunity as another eclipse crosses the country’s northern region on January 15

By Kavan Ratnatunga

Eclipses have fascinated humans through recorded history, and astrologers in Lanka have imagined two planets "rahu" and "ketu" being the head and tail of a mythical dragon to explain them.

An eclipse of the Sun, is one of those spectacular displays of nature you must watch in your lifetime. Although there are at least two solar eclipses each year somewhere on earth, they are however rare in any particular location, since the central path is less than about 250 km wide. But come January 15, 2010, those in northern Sri Lanka will be able to see another solar eclipse.

The last solar eclipse to cross Lanka was on June 20, 1955 and this was a total eclipse. It brought together the largest number of professional astronomers in Lanka. Unfortunately most of these teams located in the Polonnaruwa district were clouded out. Limited observations were done from Sigiriya, and by amateurs from Kalkudah on the East coast.

The 1955 eclipse is still remembered by all who saw it, or did not watch it because of ignorant superstition. A quack physician cum astrologer, recommended that women wanting to become fair and lovely should drink a decoction of which the main ingredient was "Vada Kaha" (Sweet Flag or Acorus Calamus) at the time of the total eclipse, preferably unseen by others. Many who took his advice ended up in hospital. The story of the "wada kaha sudiya" is remembered in a popular baila.

Eclipses can now be predicted with good accuracy for many millennia in the past and the future using details of the orbits of the Earth around the Sun and the Moon around the Earth.

The Sun is 1.4 million km in diameter and on average 150 million km distant resulting in an apparent angular diameter which varies from 31.6 to 32.7 minutes of arc. The Moon is 3.5 thousand km in diameter, and varies in distance between 406 thousand km at apogee to 356 thousand km at perigee. This means its apparent angular size ranges from 29.4 to 33.5 minutes of arc. So coincidentally the Sun and Moon are about the same size of about half-degree as observed from Earth.

A solar eclipse happens when the Moon in its orbit around the Earth passes in front of the Sun. If the apparent diameter of the Moon relative to the Sun at the time of eclipse is larger, we have a total eclipse and if smaller, we have an annular eclipse.

The next solar eclipse to cross Lanka on January 15, 2010 is an annular eclipse. The southern border (blue line on map) of the path crosses just north of Chilaw on the west coast and north of Nilavali on the East coast. The border crosses south of Anuradhapura, north of Maho. The centre-line (red) crosses Jaffna where the annular phase will have the longest duration of over 10 minutes from about 1:20 to 1:30 p.m. at an altitude of 55 degrees. About 84.2% of the centre of the Sun will be covered by the Moon, and the Sun will look like a ring of fire.

Because the Moon passes through apogee two days later (January 17 at 01:41 UT), its large distance from Earth produces an unusually wide path of annularity. The instant of greatest eclipse occurs at 07:06:33 UT when the eclipse magnitude will reach 0.919. At this location in the Indian Ocean just west of the Maldives, the duration of annularity is 11 minutes 8 seconds. Such a long annular duration will not be exceeded for over 1000 years (until December 23, 3043).

About a kilometre just north of the southern border, although the annularity will be very short, the Moon will graze the edge of the Sun, and one would observe interesting phenomena such as Baily's beads not seen further in towards the centre-line.

Weather can always be a spoiler when hoping to watch any astronomical event. Satellite images of Lanka taken around noon in January over the last three years: 2007; 2008; 2009, show that it is cloud free in the northern part of Lanka 80 to 85% of the days. In any case as long as you can see the Sun you get a good view of an annular eclipse. Unlike in a total eclipse which requires it to be perfectly clear to see the corona, that in any case is not seen in an annular.

The only time that the Sun can be viewed safely with the naked eye is during a total eclipse, when the Moon completely covers the disk of the Sun. It is never safe to look at a partial or annular eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques. Even when 99% of the Sun's surface is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause permanent retinal damage, especially when viewed through binoculars or other optical aids.

The Sun is 400,000 times brighter than the full Moon. To reduce observed brightness of the Sun to that of the full Moon, we need to filter out 99.99975% of the sunlight. This can be done using shade 14 Welders Glass, or more cheaply eclipse filters made from Mylar foil. These filters are opaque, other than when looking at the Sun. If used with an optical instrument such as a telescope or binocular the filtering MUST be at the objective BEFORE light enters the instrument.

A solar eclipse is an event which must be experienced and observed. No video can do it justice. Eclipse watching can also become addictive. I was too young to remember the 1955 eclipse and I am told it was cloudy in Colombo anyway. The first annular I saw was from USA on May 10, 1994. Then I travelled to see total eclipses on November 3, 1994 in Bolivia, October 24, 1995 in India, and in 1998, on February 26 in the West Indies.

In the 21st century the next two solar eclipses to cross Lanka on December 26, 2019 and May 21, 2031 are also annular, visible only in northern Lanka.

After that there is a total eclipse on April 11, 2070 and an annular on January 27, 2074 visible from southern Lanka, long past the lifetime of most of us.

An annular eclipse of the Sun although not as spectacular as a total eclipse is worth the effort to watch.

January 15 being a Friday and the 14th being Thai Pongal, a public holiday, what better way than for the residents of the south to travel north and celebrate Thai Pongal with our northern brethren and watch the eclipse the next day.

One problem, however, is the uncertainty over whether the A9 road to Jaffna will be open. However, within the unrestricted region south of Vavuniya, one can still get a good view of the eclipse. A more disruptive issue is current speculations of an election on the 16th which will surely be inauspicious.